1. Field of the Invention
This invention relates to a fluidized catalytic cracking (FCC) process for converting heavy vacuum gas oil and residual oil fractions into lighter products and to an apparatus therefor.
2. Description of the Related Art
Fluid Catalytic Cracking (FCC) is one of the important processes used in petroleum refineries for converting heavy vacuum gas oil into lighter products namely gasoline, diesel and liquefied petroleum gas (LPG). Processing of heavy residues e.g. atmospheric and vacuum bottoms are increasingly being practiced in the FCC Unit for enhanced conversion of residue. Heavy residues contain higher amount of conradson carbon residue CCR, poisonous metals e.g. sodium, nickel, vanadium and basic nitrogen compounds etc., all of which have significant impact on the performance of FCC unit and the stability of its catalyst.
The high CCR of the feed tends to form coke on the catalyst surface which in turn brings down its activity and selectivity. Moreover, the higher deposit of coke on the catalyst increases the regenerator temperature and therefore catalyst/oil ratio reduces for heat balanced FCC unit. The FCC catalyst can tolerate a maximum temperature of up to 750.degree. C., which limits the CCR of feed that can be processed in FCC. At present, FCC with two stage regenerators and catalyst coolers can handle up to 8 wt % feed CCR economically.
Nickel, vanadium and sodium are also available in large quantity in the residual feed. The poisoning effects of these constituents are well known in the FCC art. In the past, there have been some efforts to passivate the damaging effects of nickel and vanadium on the catalyst. These efforts have resulted only with some success in the passivation of nickel. Thus, by the known methods, it is presently possible to handle up to some 30 ppm of nickel on the feed and up to 10,000 ppm nickel on the equilibrium catalyst. Similarly, with the known processes, vanadium up to only 15 ppm on feed and 5000 ppm on the equilibrium catalyst can be handled economically. These above limits provide a serious problem of residue processing capability of FCC unit. As such, huge quantity of metal laden equilibrium catalyst are withdrawn from residue FCC unit to keep the circulating catalyst metal level within the tolerable limit. As regards the basic nitrogen compounds, suitable passivation technology is yet to be found.
In addition to the developments of passivation technologies, there have been some important design changes made for efficient residue processing. One such design change is the two stage regeneration instead of a single stage regeneration. U.S. Pat. No. 4,064,038 describes the advantages of two stage regenerator and its flexibility to handle additional feed CCR without requiring catalyst cooler. However, even with the two stage regenerator of U.S. Pat. No. 4,064,038, there is a limitation to increase feed CCR above 4.5 wt % and vanadium above 15-20 ppm on feed.
It has been suggested in the art to use a separable mixture of catalyst and inert solid particles for processing of resid. Thus, U.S. Pat. Nos. 4,895,637 and 5,110,775 suggest a physically separable mixture of FCC catalyst and vanadium additive having sufficient differences in their setting velocities so as to cover a segregation of the two types of particles in a single stage regenerator. Though such a process is simple, there are several practical disadvantages which limit its resid handling capability, namely
(i) the regenerator is kept in the dense phase where the average superficial velocity is about 0.7 meter/second. At such a velocity level, the catalyst particles still possess considerable downward gravitational pull. Moreover, there is sufficient turbulence and mixing in the bed which leads to poor segregation efficiency. PA1 (ii) It is known in the FCC art that vanadium is highly mobile in the regenerator atmosphere, and that in the single stage regenerator, the vanadium may escape from the additive to the catalyst particle. This defeats the basic purpose of catalyst/additive segregation. PA1 iii) At lower velocity of dense bed regimen, larger particles of vanadium additive may not fluidize well.
Some of these issues have been addressed by Haddam et al. in U.S. Pat. No. 4,875,994 where combustor type two stage regenerator is proposed. High velocity combustion air is used to lift the catalyst particles from the combustor. However, the mobile vanadium vapors are allowed to move to the high temperature regenerator through lift line along with the catalyst which may cause considerable damage to zeolites in the catalyst particles. In addition, the downcomer line from the regenerator to the combustor may allow the separated catalyst particle to again get mixed with the additive.
U.S. Pat. No. 4,814,068 discloses a multistage process with three sets of intermediate riser, U bend, mixing and flue gas system. Such a system is used to separate large pore catalyst particle from those having intermediate pores. The particle size of the coarse particle is also very high (500-70000 microns) to avoid the carry-over of coarse particles to the second stage regenerator.
Similarly, U.S. Pat. No 4,892,643 and 4,787,967, also take up separation of particles of two very different sizes, one having 20-150 micron and the other 500-70,000 microns. The stripper section is made annular double stage where by the difference of setting velocity of the above two size range of particles are exploited.
U.S. Pat. Nos. 4,895,636 and 4,971,766 disclose a process and apparatus for contacting residue feedstock in the dense bed kept at the riser bottom before getting cracked by the catalyst in the riser. However, the major problem is the proper atomization of feed in the dense bed with large particles at low velocity. In addition, the system will be prone to more non selective thermal cracking in the dense bed below riser resulting in higher gas and coke make. Moreover, the feed CCR will also deposit on the catalyst and therefore, the CCR related problems of residue are not addressed.
U.S. Pat. No. 4,927,522 disclose another way of increasing the residence time of ZSM-5 additive in the riser cracking process. Here the riser is made with several enlarged regions and separate feed entry locations after each enlarged section.
The inventions of U.S. Pat. No. 5,196,172 and U.S. Pat. No. 5,059,302, claim of FCC process and apparatus employing a separable mixture of catalyst and sorbent particle. Here the sorbent particles are smaller in size (30-90 microns) and the catalyst particles are bigger in size (80-150 micron). The process employs selective vortex pocket classifier and horizontal cyclone type burner to continuously separate the two types of particles.
An object of this invention is to propose a fluidized cracking process for converting heavy vacuum gas oil and residual oil fractions into lighter products and an apparatus therefor.